Nutrition and Your Health:
Dietary Guidelines for Americans

PART D: SCIENCE BASE

Section 2: Energy

This section addresses five major questions related to physical activity and energy intake:

How is physical activity related to body weight and other nutrition-related aspects of health?

How much physical activity is needed to avoid weight regain in weight-reduced persons?

What are the optimal proportions of dietary fat and carbohydrate to maintain body mass index (BMI)1 and to achieve long-term weight loss?

What is the relationship between the consumption of energy dense foods and BMI?

What is the relationship between portion size and energy intake?

The search strategies used to find the scientific evidence related to Questions 1 through 5, are shown in Appendix G-3. Questions 1, 2, and 3 have been addressed by expert panels that have published evidence-based reviews. Table D2-1 lists the BMI ranges for underweight, normal weight, overweight, and obese individuals.

The Committee conducted a literature search on three additional questions:
"Is there a level of activity below which one cannot regulate
weight?," "What is the relationship of breakfast consumption to BMI?," and
"What is the evidence to support caloric compensation for liquids versus solid foods?" The search on the first question did not result in a sufficient body of evidence to address this topic in this report. The Committee decided that the literature on the latter two questions was not sufficient to make conclusive statements, and these questions are addressed at the end of this section as Unresolved Issues.

The Committee included a strong focus on physical activity and energy expenditure in part because overweight and obesity in the United States among adults and children (Flegal et al., 2002) have increased at an alarming rate. Among adults, the prevalence of obesity has doubled in the past two decades (31 percent of adults have a BMI >30) (Flegal et al., 2002). Overweight among children has more than doubled since 1980 (7 to 16.5 percent in 1999–2002), while overweight among adolescents has tripled (5 to 16 percent in 1999-2002) (Hedley et al., 2004; Ogden et al., 2002). Information on differences in the prevalence of obesity by racial/ethnic group appears in Part B, the Introduction.

QUESTION 1: HOW IS PHYSICAL ACTIVITY RELATED TO BODY WEIGHT AND OTHER NUTRITION-RELATED ASPECTS OF HEALTH?

Conclusions

Regular physical activity is essential to the maintenance of a healthy weight
and reduces risk for the development of a number of chronic diseases. At least
30 minutes of moderate physical activity on most days provides important health
benefits in adults. More than 30 minutes of moderate to vigorous physical activity
on most days provides added health benefits. Many adults may need up to 60 minutes
of moderate to vigorous physical activity on most days to prevent unhealthy weight
gain.

Vigorous physical activity (e.g., jogging or other aerobic exercise) provides greater benefits for physical fitness than does moderate physical activity and burns calories more rapidly per unit of time.

Exercise that loads the skeleton has potential to reduce the risk of osteoporosis by increasing peak bone mass during growth, maintaining peak bone mass during adulthood, and reducing the rate of bone loss during aging.

Resistance exercise training increases muscular strength and endurance and maintains or increases lean body weight. These benefits are seen in adolescents, adults, and older adults who perform 8 to 10 resistance exercises 2 or more days per week.

Children and adolescents need at least 60 minutes of moderate to vigorous physical activity on most days for maintenance of good health and fitness and for healthy weight during growth. Reducing sedentary behaviors, including television and video viewing time, appears to be an effective way to treat and prevent overweight among children and adolescents.

Rationale

Adults

These conclusions are based on the Committee's systematic review of 36 longitudinal
studies and 2 intervention studies addressing this issue (see Appendix G-3).

Physical Activity and the Prevention of Excessive Weight Gain

Overweight and obesity result from inadequate physical activity and/or excess
calorie consumption. A sedentary lifestyle is a lifestyle characterized by little
or no physical activity. Data suggest that physical activity levels are low for
most Americans. For example, beyond the light activity of day-to-day living,
in 2002, 38 percent of adult Americans engaged in no leisure-time physical activity
(NHIS, 2002).

Thirty of 36 longitudinal studies (Appendix G-3) show an inverse relationship between physical activity and overweight status. Of six of the remaining longitudinal studies, five show no significant relationship between physical activity and weight status, and one (Bild et al., 1996) found an increase in body weight associated with a large amount of vigorous physical activity at baseline.2

The role of physical activity in the prevention of weight gain was studied in a systematic review of 16 observational studies (Fogelholm and Kukkonen-Harjula, 2000). In a separate systematic review, Erlichman and colleagues (2002) included studies only if they could estimate the Physical Activity Level equivalent (PAL) of the specified activities as a means of standardizing their approach. An expert panel convened by the International Association for the Study of Obesity (IASO)3 reviewed the evidence presented by Erlichman et al. (2002). That panel concluded that approximately 45 to 60 minutes of moderate intensity daily physical activity is needed to prevent the transition from a healthy weight to overweight or from overweight to obesity (Saris et al., 2003).4 Table D2-2 gives examples of moderate physical activities.

Based on an extensive compilation of cross-sectional doubly-labeled water studies in humans, the Institute of Medicine (IOM)(2002) reported that two-thirds of adults who maintain energy balance expended an equivalent amount of energy to that which would be expended by engaging in 60 minutes per day at moderate intensity. Thus, it appears that many adults need up to 60 minutes per day of at least moderate intensity physical activity to prevent unhealthy weight gain (IACR,5 2002; IOM,6 2002; Saris et al., 2003).

Physical Activity and Other Aspects of Health

The consensus public health recommendation for physical activity in adults—
at least 30 minutes of at least moderate intensity physical activity daily—was
developed with a primary focus on the chronic disease risk reduction and fitness
enhancement effects (HHS, 1996; Pate et al., 1995). Most authorities also acknowledge
that vigorous physical activity for at least 20 minutes on at least 3 days per
week is another appropriate way to perform physical activity for health and fitness
(ACSM,7 1998). Examples of vigorous physical activities
appear in Table D2-3. The health benefits of regular physical activity include
the reduction of risk of a number of chronic conditions and diseases that relate
to diet as well. Among these are high blood pressure, stroke, coronary artery
disease, type 2 diabetes, colon cancer, and osteoporosis. (Pate et al., 1995;
Shephard, 2001).

Decreases in blood pressure and the prevention of stroke seem best achieved by a moderate rather than a high intensity of physical activity (Shephard, 2001). Vigorous intensity seems necessary to augment bone health (HHS, 1996; Pate et al., 1995). Although some health benefits are dependent on the intensity of physical activity (i.e., moderate or vigorous), most aspects of metabolic health depend on the total volume of activity. That is, vigorous physical activity can have greater effects than moderate physical activity of the same duration, but it is the combination of intensity (moderate or vigorous) and the duration of this activity that affects both caloric expenditure and overall health (Kesaniemi et al., 2001; Shephard, 2001).

Physical inactivity is an independent risk factor for atherosclerotic cardiovascular disease, type 2 diabetes, colon cancer, and other chronic diseases (American Heart Association, 1992; HHS, 1996). Increases in physical activity are associated with reduced risk of chronic disease and mortality from all causes (Blair et al., 1995; Paffenbarger et al., 1993), and this effect is mediated by numerous physiological adaptations including improvements in weight status and body composition. However, the health effects of physical activity and physical fitness are not explained primarily by its effect on body weight. Overweight persons derive important health benefits from maintaining good levels of physical activity and physical fitness (Lee et al., 1999).

Resistance Exercise Training

Resistance exercise (e.g., weight training, using weight machines, callisthenic
exercises, and resistance band activities) increases muscular strength and maintains
or increases lean muscle mass in persons of all ages (ACSM, 2002). In the elderly,
resistance exercise assists in balance and locomotion, thereby reducing the risk
of falling (Evans, 1999). The health benefits of resistance exercise accrue to
those who perform, on 2 or more days per week, 1 or more sets of 8 to 10 exercises
that condition the major muscle groups.

Exercise and Bone Health

Building maximal peak bone mass during growth and minimizing the loss of bone
during one's later years are strategies for reducing the risk of fracture. Rapid
accrual of bone mass occurs during puberty (Bailey et al., 1999) and continues
throughout adolescence and into young adulthood (Heaney et al., 2000). Approximately
20 to 50 percent of the variation in bone mass is thought to be modified by lifestyle
choices including physical activity and nutrition.

Bone adapts to the loads applied to it (Frost, 1990; Rubin and Lanyon, 1985). The skeletal response to exercise is greatest in people who are least active. Loading can occur by gravitational forces and muscle pull A systematic review of weight-bearing physical activity intervention studies in children and adolescents showed a positive effect on bone mass (French et al., 2000). Pediatric studies relating exercise and bone gain reviewed by the Committee are summarized in Appendix G-3. Thirteen of 15 physical activity intervention trials in children show a positive effect of exercise intervention on one or more bone sites. Exercise interventions have greater impact on bone mass if initiated during prepubertal years than later in life. Perhaps even more importantly, in children exercise can lead to changes in bone geometry that can confer greater strength (Bass et al., 2001; Haapasalo et al., 1996; Specker and Binkley, 2003).

A physically active lifestyle that includes regular participation in weight-bearing exercise is beneficial to weight management, fitness, and bone health. Hip fracture incidence was 30 to 50 percent lower in adults with a history of daily physical activity than in sedentary individuals (Coupland et al., 1993; Cummings et al., 1995; Farmer et al., 1989). For example, in 9,704 women over age 65 participating in the Study of Osteoporosis Fracture, the incidence of fracture over 7.6 years was 27 percent lower with low-intensity activity and 45 percent lower with moderate physical activity (Gregg et al., 1998).

Specific recommendations for the type, frequency, intensity, and duration of exercise should be individualized with respect to lifestage and health (Singh. 2004). Because the effects of loading are site specific and load dependent (Kerr et al., 1996), the most effective types of physical activity for bone health are weight-bearing exercises such as jogging, walking, aerobics, stair climbing, and strength training (Kohrt et al., 1997; Nelson et al., 1991; Snow-Harter et al., 1992). Extreme exercise that leads to growth plate injury or estrogen-deficiency associated with amenorrhea is detrimental to bone (Forwood and Burr, 1993).

Children

Two types of evidence are available related to physical activity and weight
status in children: considerations of amounts of physical activity consistent
with a healthy body weight and studies of sedentary activity (mainly television
viewing).

Increasing Physical Activity

Although the relevant scientific literature is limited, most expert panels
have come to consensus that children and youth need at least 60 minutes of moderate
to vigorous physical activity per day on most days of the week to help promote
healthy growth and development and to help avoid unhealthy weight gain (Cavill
et al., 2001). This recommendation considers the increasing prevalence of overweight
among children and their current physical activity levels: most children and
youth already engage in 30 minutes of physical activity daily, but many do not
meet a 60-minute standard (Biddle et al., 1998; Kimm et al., 2002).

Television Viewing

The average child or adolescent watches nearly 3 hours of television per day,
not including time spent watching videotapes or playing video games (Nielsen,
1998). A 1999 study of a large nationally representative sample found that 2-
to 18-year-olds spend an average of 5 hours and 29 minutes per day with various
media combined (Roberts et al., 1999). The prevalence of overweight has been
shown consistently to be directly related to the amount of time children and
adolescents watch television (Andersen et al., 1998; Berkey et al., 2000; Deheeger
et al., 1997; Dietz and Gortmaker, 1985; DuRant et al., 1994; Gortmaker et al.,
1996; Grund et al., 2001; Guillaume et al., 1997; Hanley et al., 2000; Hernandez
et al., 1999; Maffeis et al., 1998; Muller et al., 1999; Ross and Pate, 1987,
Sallis et al., 1995); and reductions in television and video viewing time appear
to be effective strategies to treat and prevent overweight. One school-based
study demonstrated a 2 percent decrease in the prevalence of overweight over
the course of two school years as a result of a curriculum that focused on reduced
television viewing time (Gortmaker et al., 1999). A second school-based study
demonstrated reduced rates of weight gain in children who reduced television
time (Robinson, 1999). The American Academy of Pediatrics recommends limiting
television and video viewing to a maximum of 2 hours per day as a strategy to
prevent overweight in children (AAP, 2003).

Pregnancy

Epidemiologic data suggest that physical activity may be beneficial in the
primary prevention of gestational diabetes, particularly in pregnant women with
a prepregnancy BMI > 33 (ACOG, 2002; Dempsey et al., 2004; Dye et al., 1997).
Rössner (1999) reported smaller increase in skinfold measurements in pregnant
women who exercised, indicating less gain in body fat by those who exercised
than by those who did not. The physiologic and morphologic changes of pregnancy
may interfere with a woman's ability to engage safely in some forms of physical
activity. Activities with a high risk of falling or of abdominal trauma should
be avoided during pregnancy. In the absence of either medical or obstetric complications,
30 minutes or more of moderate physical activity per day on most, if not all,
days of the week is recommended for pregnant women (ACOG, 2002).

Lactation

Dewey et al. (1991) have shown that the level of physical activity of the nursing
mother does not affect lactation. Neither acute nor regular exercise has adverse
effects on a mother's ability to successfully breastfeed successfully (Larson-Meyer,
2002).

Older Adulthood

Participation in a regular program of physical activity is an effective method
to reduce a number of declines in function that are associated with aging and
can help in the management of weight and constipation and the prevention of osteoporosis.
Endurance training can help maintain and improve various aspects of cardiovascular
function. Strength training helps offset the loss in muscle mass and strength
typically associated with aging. Even octo- and nonagenarians have demonstrated
the ability to adapt to both endurance and strength training. Strength training
can improve bone health, increase muscle mass, and improve postural stability
thus reducing the risk of falling and associated injuries and fractures (ACSM,
1998).

QUESTION 2: HOW MUCH PHYSICAL ACTIVITY IS NEEDED TO AVOID WEIGHT REGAIN IN WEIGHT-REDUCED PERSONS?

Conclusions

Although the contribution of physical activity to weight loss usually is modest,
acquiring a routine of regular physical activity will help an adult to maintain
a stable body weight after successful weight loss. The amount of physical activity
that weight-reduced adults need to avoid weight regain is estimated to be from
60 to 90 minutes daily at moderate intensity.

Rationale

This conclusion is based on the Committee's review of cross-sectional data
from the National Weight Control Registry, two metabolic studies using the doubly-labeled
water (DLW) technique, and a published extensive systematic review of observational
studies and randomized clinical interventions. Overall, studies have shown that
individuals who follow a regular regimen of physical activity after they lose
weight are much more likely to maintain their lower weight than those who rely
only on diet control, as described below (see Appendix G-3 for a summary of relevant
studies).

Cross-sectional data from the National Weight Control Registry show that individuals who have maintained a weight loss of approximately 30 kg for about 6 years participate in a large amount of leisure-time physical activity (2,545 kcal per week of physical activity for women and 3,293 kcal per week for men) (McGuire et al., 1999). This amount of physical activity is comparable to about 60 to 90 minutes per day of moderate intensity physical activity, such as brisk walking (Wing and Hill, 2001). The reported calorie expenditure of the weight maintainers was 450 kcal per day more than that of the persons who had regained the weight they had lost (P=0.02), showing that the inclusion of a physical activity regimen helps to maintain reduced weight.

Metabolic studies using the DLW technique can provide useful estimates of an individual's level of physical activity. Using this approach to estimate the physical activity levels of a group of 32 women after weight loss, Schoeller and colleagues (1997) reported that weight was maintained for 1 year when the subjects averaged the equivalent of 80 minutes of moderate activity every day. Another study using DLW to estimate the physical activity level reported similar results: the weight-reduced subjects maintained their weight for 1 year when they engaged in moderate activity for 77 minutes per day, but those who engaged in much less physical activity regained weight (Weinsier et al., 2002). The results of these studies are consistent with the findings from the National Weight Control Registry reported above.

The role of physical activity in the prevention of weight regain was studied in a systematic review both of observational studies and randomized clinical interventions (Fogelholm and Kukkonen-Harjula, 2000). Nineteen studies with a nonrandomized weight reduction phase and an observational followup were reviewed. Of these, 16 studies found an inverse relationship between physical activity and weight regain, and 3 found no significant relationship. The design of several of these studies (Ewbank et al., 1995; Hartman et al., 1993; Schoeller et al., 1997) allowed estimation of the difference in energy expenditure of the low- and high-exercise groups. In particular, the difference ranged from 1,300 to 2,000 kcal per week. The low-activity group gained approximately 5 to 8 kg more per year than did the high-activity group. Fogelholm and Kukkonen-Harjula (2000) also reviewed reports of three interventions involving physical activity during the weight maintenance phase. The results were inconsistent. Leermakers et al. (1999) reported that the exercise group gained more weight than the weight-focused group. Fogelholm et al. (2000) found that the moderate walking group gained less weight than the control group, but the heavy walking group did not differ from the control group. Perri et al. (1989) reported that the weight regain of the extended treatment group did not differ from that of the standard group at 20 weeks, but the extended treatment group showed significantly greater mean weight loss at 40 and 72 weeks. Since the weekly amount of prescribed physical activity in these trials varied from 80 to 300 minutes per week (about 11 to 43 minutes per day), the amount of physical activity may have been too small to have a statistically significant effect on weight maintenance. When looking at the full body of evidence, Fogelholm and Kukkonen-Harjula (2000) concluded that the physical activity equivalent of 1,500 to 2,000 kcals per week is associated with weight maintenance. This range of calories is equivalent to approximately 60 to 90 minutes of moderate physical activity per day.

Finally, an expert panel convened by the International Association for the Study of Obesity reviewed the existing studies, including the systematic review by Fogelholm and Kukkonen-Harjula (2000). The panel concluded,
"there is compelling evidence that prevention of weight regain in formerly obese individuals requires 60 to 90 minutes of moderate intensity activity or lesser amounts of vigorous intensity activity" (Saris et al., 2003).

Thus, a broad range of evidence supports a recommendation that weight-reduced persons take part in 60 to 90 minutes of moderate physical activity daily to maintain their lower body weight and avoid regain of weight. This is a longer duration of physical activity than is needed by never-obese persons to avoid weight gain.

QUESTION 3: WHAT ARE THE OPTIMAL PROPORTIONS OF DIETARY FAT AND CARBOHYDRATE TO MAINTAIN BMI AND TO ACHIEVE LONG-TERM WEIGHT LOSS?

Conclusions

Weight maintenance depends on a balance of energy intake and energy expenditure,
regardless of the proportions of fat, carbohydrate, and protein in the diet.
Weight loss occurs when energy intake is less than energy expenditure, also regardless
of the proportions of fat, carbohydrate, and protein in the diet. For adults,
well-planned weight loss diets that are consistent with the Accepted Macronutrient
Distribution Ranges (IOM, 2002) for fat, carbohydrate, and protein can be safe
and efficacious over the long term. The recommended ranges for fat calories (20
to 35 percent of total calories), carbohydrate calories (45 to 65 percent of
total calories), and protein calories (10 to 35 percent of total calories) provide
sufficient flexibility to accommodate weight maintenance for a wide variety of
body sizes and food preferences.

Rationale

These conclusions are based on the Committee's consideration of short- and
long-term intervention studies reviewed by an expert IOM Committee (IOM, 2002).
Additionally, this Dietary Guidelines Advisory Committee conducted a systematic
review of the scientific literature published since 1999 (after the conclusion
of the IOM review). The search covered intervention and longitudinal studies,
and the results included 12 clinical trials and 3 observational studies. (See
Table D2-4 for intervention studies up to the year 2000; and see Appendix G-3
for a summary of relevant results of the search of publications after 1999.)

Weight Reduction

Background Information. A sound long-term weight loss plan includes a reduction of caloric intake, the intake of recommended amounts of nutrients, and increased physical activity. Lifestyle change in diet and physical activity is the best first choice for weight loss. (See
"Supplemental Information—Scientific Support for Weight Loss and Weight Management Recommendations" below for Federal guidelines for weight reduction.)

Diets balanced in macronutrients have traditionally been recommended for weight loss (American Heart Association, 2001; Frantz et al., 2002; NIH,NHLBI, 1998; St. Jeor et al., 2001). Numerous studies attest to their efficacy (Diabetes Prevention Program Research Group, 2002; Tuomilehto et al., 2001; Wing and Hill, 2001). However, many persons are going on very-low carbohydrate or very-low fat diets. These popular weight-loss diets encompass a very wide range of carbohydrate/fat ratios, ranging from less than 10 percent of calories from fat to more than 50 percent of calories from fat. They have not been tested adequately over the long-term and are best followed only for short periods of time.

Low-Carbohydrate, High-Fat Diets. The propounded theory behind
low-carbohydrate, high-fat diets is that a drastically reduced carbohydrate intake
will lower insulin levels, allow uninhibited lipolysis, increase fat oxidation,
initiate ketone production, and decrease appetite (Atkins, 1999). Another expectation
of diets with an extremely low ratio of carbohydrate to fat is that they will
facilitate compliance and increase water losses. Five randomized controlled trials
(Brehm et al., 2003; Fleming, 2002; Foster et al., 2003; Samaha et al., 2003;
Westman et al., 2002) recently have compared weight loss after 6 months to a
year on diets that have low carbohydrate-to-fat ratios with weight loss on more
balanced diets. The low-carbohydrate diets initially provided less than 20 to
30 g of carbohydrate per day (followed by 40 to 60 g of carbohydrate per day
after the first 2 weeks in both Brehm et al. (2003) and Foster et al. (2003)).
Control diets provided 60 percent of calories from carbohydrate, 25 to 30 percent
of calories from fat, and 15 percent of calories from protein (Brehm et al.,
2003; Fleming, 2002; Foster et al., 2003; Samaha et al., 2003; Westman et al.,
2002). All studies found that the low-carbohydrate diets produced greater initial
weight loss, but the difference was modest. For example, Foster and colleagues
(2003) reported that mean weight loss at 6 months was 7.0 percent below baseline
for those on the low-carbohydrate diet compared with 3.2 percent below baseline
for those on the control diet. At 18 months, however, there was no statistically
significant difference in weight loss. Some of the early weight loss on a low-carbohydrate
diet is due to water loss (Yang and Van Itallie, 1976; Bortz et al., 1967). Whether
the remaining difference in initial weight loss is due to a lower energy intake,
a larger energy expenditure, or a combination of the two is not known. In any
case, differences in weight loss tend to diminish, and by 12 to 18 months no
real difference remains.

The long-term safety of any diet needs to be considered. Unfortunately, only short-term data (6 to 12 months) are available for these diets. Within this period of followup, no evidence of serious adverse effects has been published. However, the diets require that dietary supplements be taken regularly because the diets are low in vitamins E, A, thiamin, B6, and folate; calcium; magnesium; iron; potassium; and dietary fiber (Freedman et al., 2001). Very-low-carbohydrate diets often include a high percentage of protein along with the high percentage of fat. Usually, this includes large amounts of animal protein, which adds substantially to the saturated fat and cholesterol intake. A recent study has cautioned that such diets also can lead to a high urinary calcium loss and kidney stones (Reddy et al., 2002). Uric acid production is increased and may lead to elevated blood uric acid concentrations. There are very few long-term trials of high-protein weight loss diets. Skov et al. (1999) showed a greater weight loss with a higher protein diet (25 percent of total energy) than with a lower protein diet (12 percent of total energy) (loss of 8.9 kg and 5.1 kg, respectively) over 6 months. Another study, 10 weeks long, showed no difference in the body composition, cholesterol, triglycerides, uric acid, percent body fat, or nutrient intake in sedentary, overweight women following 1,200 calorie diets with varying macronutrient distributions (Alford et al., 1990). Interestingly, blood lipid values in the various studies of high-fat diets were found to have improved at least as much as in the lower-fat control diets (Foster et al., 2003; Samaha et al., 2003). LaRosa et al. (1980), however, reported an increase in serum low-density lipoprotein (LDL) cholesterol on a high-protein/high-fat diet.

The concern regarding the long-term safety of high-fat, low-carbohydrate diets is warranted given that (1) they have a high saturated fat, high cholesterol, and low fiber content;8 (2) they result in a very low intake of fruits, vegetables, and grains (which could lead to deficiencies in essential vitamins, minerals, and fibers over the long-term); and (3) they originally were designed for short-term use during a weight loss period and have not been evaluated long-term.

High-Carbohydrate, Low-Fat Diets. A diet with a high-carbohydrate/fat
ratio (that is, a very low-fat diet) has been popularized by Ornish (1990) and
Pritikin (1988). This diet suggests decreasing fat intake to about 10 percent
of calories, keeping protein at 15 percent of calories, and eating about 75 percent
of calories as carbohydrates. The high-carbohydrate content is compatible with
achieving more than the recommended intake of fruits, vegetables, and fiber.
However, the very-low fat content may increase the risk of essential fatty acid
deficiency (IOM, 2002) and may reduce the bioavailability of some fat-soluble
vitamins (IOM, 2002; Roodenburg et al., 2000). In a weight-loss study Mueller-Cunningham
et al. (2003) prescribed a diet with less than 15 percent of total calories from
fat and reported a decrease in the intakes of vitamin E (as α-tocopherol) and
of n-3 fatty acids. Freedman et al. (2001) described these high-carbohydrate/low-fat
diets as being low not only in vitamin E, but also in vitamin B12
and zinc. The other negative consequence of a low-fat diet is that it usually
is a high-carbohydrate diet, which can lead to increased levels of triglycerides
(see Part D, Section 4, "Fats").

Weight Maintenance

For weight maintenance, the desirable diet is one that prevents weight gain,
meets nutrient needs, and can be consumed for a long time without adverse effects.
One of the questions is how much fat should be in such a diet. The majority of
observational studies and surveys support an association between dietary fat
intake and BMI. Bray and Popkin (1998) summarized data from a variety of populations
in more than 20 countries and reported an association between greater fat intake
and higher BMI. However, Willett (1998) points out that this relationship is
not consistent across countries and that the effect of fat intake on BMI is rather
minor.

For adults, the Acceptable Macronutrient Distribution Ranges (AMDRs) for fat, protein, and carbohydrate are estimated to be 20 to 35 percent, 10 to 35 percent, and 45 to 65 percent of energy, respectively (IOM, 2002). The upper range for fat, 35 percent of total calories, is based on the increased risk of overconsuming calories and of obesity with fat intakes above that range (Astrup et al., 2000; Saris et al., 2000; Shepard et al., 2001; Tremblay et al., 1991). Thus, diets with very-low carbohydrate to fat ratios (i.e., diets high in fat) may not be desirable for weight maintenance. The lower limit of fat recommended 20 percent of calories and aims at avoiding (1) fatty acid deficiency when fat intake is too low (Mueller-Cunningham et al., 2003), and (2) excess carbohydrate intake, which may have adverse effects on the blood lipid profile (see Part D, Section 4,
"Fats").

Both the low-carbohydrate diet and the low-fat diet limit the variety of foods that can be eaten and, therefore, may be difficult to follow long-term (Foster et al., 2003). This probably explains the extremely high dropout rates in studies of these diets. There is insufficient evidence to make recommendations for or against the use of these diets for weight loss, but there is great concern about their long-term use for weight maintenance (Bravata et al., 2003).

Although both low-fat diets and low-carbohydrate diets have been shown to result in weight reduction if followed, the maintenance of a reduced weight ultimately will depend on a change in lifestyle from the one that resulted in the need for weight reduction to one that meets nutrient needs while maintaining a balance between energy consumption and energy expenditure (Freedman et al., 2001).

Special Groups

Pregnant Women. Weight gain rather than weight maintenance or weight loss is indicated for pregnant women. The IOM has recommended the following gains in weight for women according to their prepregnancy BMI: (1) underweight (BMI <19.8), 28 to 40 pounds; (2) normal weight (BMI 19.8–26.0), 25 to 35 pounds; (3) overweight (BMI 26–29), 15 to 25 pounds; (4) obese (BMI >29), at least 15 pounds (IOM, 1990). For the obese woman, the amount of weight gain should not exceed 20 pounds. It is important for the pregnant woman to get adequate protein (71 g per day) (IOM, 2002). A low-protein intake during pregnancy is associated with a higher incidence of low-birth-weight infants and should be avoided (IOM, 2002). However, taking too much protein also is unwise. Randomized controlled studies have shown that supplementary protein can decrease birth weight and increase mortality (Rush et al., 1980; Sloan et al., 2001). In addition, the Recommended Dietary Allowance (RDA) for carbohydrate for pregnant women is 175 g per day (IOM, 2002), and is important for prevention of hypoglycemia. Thus, a low-carbohydrate high-protein diet is not appropriate during pregnancy. AMDRs for protein and carbohydrate intake for pregnant women are 10 to 35 percent and 45 to 65 percent, respectively (IOM, 2002).

Lactation. Moderate weight reduction while breastfeeding is
safe and does not compromise weight gain of the infant (ACOG, 2002). The RDA
for protein for breastfeeding women is 71 g per day (IOM, 2002). The RDA for
carbohydrate increases during lactation to 210 g per day (IOM, 2002). AMDRs for
protein and carbohydrate intake for breastfeeding women are 10 to 35 percent
and 45 to 65 percent respectively (IOM, 2002).

SUPPLEMENTARY INFORMATION—SCIENTIFIC SUPPORT FOR WEIGHT LOSS AND WEIGHT

Management Recommendations

The National Heart, Lung, and Blood Institute, in cooperation with the National
Institute of Diabetes and Digestive and Kidney Diseases, released the first Federal
guidelines on the identification, evaluation, and treatment of overweight and
obesity using an evidence-based model and methodology (NIH, NHLBI, 1998). The
guidelines present recommendations for the assessment of overweight and obesity
and establish principles of safe and effective weight loss.

The guidelines' definition of overweight is based on research that relates BMI to the risk of death and illness. The 24-member expert panel that developed the guidelines identified overweight as a BMI of 25 to 29.9 and obesity as a BMI of 30 and above, which is consistent with the definitions used in many other countries. BMI describes body weight relative to height and is strongly correlated with total body fat content in adults. According to the guidelines, a BMI of 30 is about 30 pounds overweight and is equivalent to 221 pounds in a 6' person and to 186 pounds in someone who is 5'6". The BMI values apply to both men and women. Some very muscular people may have a high BMI without health risks, but they represent a very small percentage of the population.

Also recommended in the guidelines is the determination and tracking of waist circumference, which is strongly associated with abdominal fat. Excess abdominal fat is an independent predictor of disease risk. A waist circumference of over 40 inches in men and over 35 inches in women signifies increased risk in those whose BMI is 25 to 34.9.

According to the guidelines, the most successful strategies for weight loss include calorie reduction, increased physical activity, and behavior therapy designed to improve eating and physical activity habits. Recommendations regarding the goal and rate of weight loss follow:

The initial goal of treatment should be to reduce body weight by about 10 percent from baseline, an amount that reduces obesity-related risk factors. With success, and if warranted, further weight loss can be attempted.

A reasonable timeline for a 10 percent reduction in body weight is 6 months of treatment, with a weight loss of 1 to 2 pounds per week.

QUESTION 4: WHAT IS THE RELATIONSHIP BETWEEN THE CONSUMPTION OF ENERGY DENSE FOODS AND BMI?

Conclusions

Available data are insufficient to determine the contribution of energy dense
foods to unhealthy weight gain and obesity. However, consuming energy dense meals
may contribute to excessive caloric intake. Conversely, eating foods of low energy
density may be a helpful strategy to reduce energy intake when trying to maintain
or lose weight.

Rationale

This conclusion is supported by the Committee's review of six short-term studies,
one longitudinal study, and two longer-term randomly controlled trials, as summarized
below.

The energy density of a food (kcal/100 g) depends on its content of fat, carbohydrate, protein, and water. Of particular importance are the content of fat (which provides twice the calories per g compared to carbohydrate and protein) and of water (which provides no calories). The air content of foods contributes to their volume rather than to their energy density.

Short-Term Studies

Short-term studies (ranging in length from a single test meal to 5 days of
feeding) consistently demonstrate that the ad libitum consumption of
foods results in significantly higher total energy intakes when the food offered
is high in energy density than when it is low in energy density (Bell et al.,
1998; Bell and Rolls, 2001; Duncan et al., 1983; Rolls et al., 1999a; Stubbs
et al., 1998). In a study by Duncan et al. (1983), satiety ratings from low energy
density (LED) and high energy density (HED) meals were compared in a group of
obese and nonobese subjects. Individuals on the LED diet reached satiety at a
mean daily energy intake that was one-half that of the mean daily energy intake
of the individuals on the HED diet (1,570 versus 3,000 kcal). This higher intake
of energy for those consuming HED meals ad libitum has been attributed
to a delay in the development of satiety with more energy-dense foods (Duncan
et al., 1983; Rolls et al., 1999a). In the Duncan study, the energy density of
the HED diet was approximately twice that of the LED diet. Thus, in consuming
half the mean daily energy intake of the HED, those on the LED consumed roughly
the same amount of food (by weight) as those on the HED diet.

In the studies discussed in the above paragraph, the LED and HED diets varied from each other in macronutrient distribution. Results similar to those reported above were obtained in studies in which energy density was manipulated without altering the macronutrient distribution. This was achieved through covert changes in energy density (Rolls et al., 1999a; Stubbs et al., 1998) or by increasing the water content of foods (Rolls et al., 1999b). Therefore, the short-term effects of energy density on satiety, total energy intake, and body weight are not necessarily dependent on the fat or carbohydrate content or the percentage of fat or carbohydrate calories in the meal. In most studies, protein and fiber are held constant.

The lower the energy density of a food, the higher the amount (by weight) of food that needs to be consumed to reach a given caloric intake. To discriminate between the effects of energy density and food volume, Rolls and colleagues (2000a) manipulated food volume by adding variable amounts of air to test meals of identical macronutrient composition and energy content. This study demonstrated that higher-volume meals significantly reduce energy intake, even when the macronutrient distribution is unchanged. In this case, the study focused on volume, and showed that for total energy intake both mass and volume are important.

Randomized Controlled Trials

Two longer-term, randomized controlled trials involving overweight individuals
provide useful information regarding the satiety and compensatory effects of
diets with different energy densities. Although these studies were not conducted
to investigate the effect of energy density on caloric consumption specifically,
the energy density in the test foods was manipulated, and so the results are
useful in this discussion.

In a 9-month ad libitum study, Lovejoy and colleagues (2003) replaced one-third of the fat calories with the fat substitute Olestra®, which provides no calories. This study showed that the lower-density Olestra treatment resulted in a weight loss of 6.27 kg during the study period, compared with 4.0 kg in the control group (p = 0.06).

A 10-week ad libitum food-intake study by Raben et al. (2002) supplemented a standard diet with sweetened drinks and foods. The foods for the control group were sweetened with sucrose, and the foods for the experimental group were artificially sweetened. The result was a significant difference (p = < 0.001) in body weight: the control group gained an average of 1.6 kg, whereas the artificial sweetener group lost an average of 1.0 kg.

Other Studies

One cross-sectional study found that the consumption of energy-dense, nutrient-poor
foods was a predictor of being overweight (Nicklas et al., 2003), but intake
of the foods with low nutrient density explained less than 5 percent of the variance
in overweight status. This relationship between the consumption of energy dense,
nutrient-poor foods and weight was not confirmed by others (Bandini et al., 1999;
Kant, 2003).

In summary, short-term studies have linked energy density with total energy intake over a period of 1 meal to 5 days. While not specifically performed to investigate the effects of energy density on satiety, two longer randomized trials showed that, compared with diets of high energy density, diets low in energy density resulted in a weight loss relative to the control group. However, evidence that the consumption of energy dense foods contributes to a change in BMI is still lacking.

QUESTION 5: WHAT IS THE RELATIONSHIP BETWEEN PORTION SIZE AND ENERGY INTAKE?

Conclusion

The amount of food offered to a person influences how much he or she eats;
and, in general, more calories are consumed when a large portion is served rather
than a small one. Thus, steps are warranted for consumers to limit the portion
size they take or serve to others, especially for foods that are energy dense.

Rationale

These conclusions are supported by the Committee's review of six short-term
feeding studies, one longitudinal study, and three observational studies, as
described below.

Short-Term Studies

Studies using a short-term ad libitum intake model demonstrate that
serving larger portions results in a larger volume of food consumed and a higher
energy intake (Diliberti et al., 2004; Fisher et al., 2003; Rolls et al., 2002,a
2004a). In a study of 51 men and women, these results occurred whether the portion
served was placed on the individual's plate or was selected by the individual
from a serving dish (Rolls et al., 2002a). The response of 5-year-old children
to portion size appears to be similar to that of adults: increased energy intake
from larger portion sizes (Rolls et al., 2000b). This study showed that children
younger than 3 years consumed similar volumes of food when served different portion
sizes; but, by age 5, they increased their intake when served larger portions
(Rolls et al., 2000a). Another study by the same group showed that large portion
sizes have different effects on energy intake in children age 3 to 5 depending
on whether the food is served on individual plates or the children serve themselves
from a serving dish (Fisher et al., 2003). When children served themselves, they
spontaneously controlled their portion size and consumed similar amounts of energy
from large and small serving dishes.

Other Studies

The Committee's search did not identify any randomized controlled trials evaluating
the role of portion size on energy intake or BMI (Hannum et al., 2004). One longitudinal
study in children reported a positive relationship (p < 0.05) between portion
size and body weight (McConahy et al., 2002). Several other observational studies
have reported that a secular increase in portion size coincides with the rise
in obesity in the United States over the past decades (Nielsen and Popkin, 2003;
Smickilas-Wright et al., 2003; Young and Nestle, 2002, 2003).

Overall, the evidence supports the conclusion that servings that are too large may be part of the
"obesogenic" environment, inasmuch as they facilitate excess consumption of energy.

UNRESOLVED ISSUES

What Is the Relationship Between Breakfast Consumption and BMI?

One randomized clinical trial (RCT) and two longitudinal studies in the literature were reviewed. The purpose of the RCT was to study the effect of eating or not eating breakfast on the outcome of a weight-loss trial (Schlundt et al., 1992). The breakfast group ate 3 meals a day and the no-breakfast group ate 2 meals a day. The energy content of the two diets was identical. There was no significant difference in weight loss at 12 weeks.

Two longitudinal studies, one in children (Berkey et al., 2003) and one in adults (Ma et al., 2003), provide relevant data. Berkey et al. (2003) studied more than 14,000 children age 9 to 14 years in 1996, using data from mailed questionnaires. Overweight children who never ate breakfast lost more body fat over the year of followup than overweight children who ate breakfast nearly every day; however, normal weight children who never ate breakfast gained weight comparable to that of normal weight children who ate breakfast nearly every day. Thus, this study is inconclusive.

The Seasonal Variation Blood Cholesterol Study conducted in 1994–1998 evaluated the relationship between eating patterns and obesity. Odds ratios were adjusted for other obesity risk factors including age, sex, physical activity, and total energy intake. A greater number of eating episodes per day was associated with a lower risk of obesity (odds ratio for 4 or more eating episodes versus 3 or fewer episodes was 0.55, 95 percent confidence interval: 0.33, 0.91.) In contrast, skipping breakfast was associated with an increased prevalence of obesity (odds ratio = 4.5, 95 percent confidence interval: 1.57, 12.90) (Ma et al., 2003).

Information from the U.S. National Weight Loss Registry indicates that eating breakfast is an important factor in maintaining weight-loss over time (Wyatt et al., 2002).

Using data from the 1977–1978 Nationwide Food Consumption Survey, Morgan and colleagues reported that skipping breakfast lowered the nutritional quality of the diets of adults (Morgan et al., 1986) and of the elderly (Morgan and Zabik, 1984).

Thus, there is suggestive evidence from cross-sectional studies and longitudinal studies that eating breakfast is likely to promote healthy weight and improve the nutritional quality of the diet, but more studies are needed before a definitive conclusion can be reached. However, while the evidence is inconclusive that eating breakfast may help to manage body weight, eating breakfast regularly does not increase the risk of gaining weight. Therefore, adults and children should not skip breakfast because of concerns that breakfast leads to overweight or obesity. Additionally, skipping breakfast may lower the nutritional quality of the diet.

What Is the Evidence To Support Caloric Compensation for Liquids versus Solid Foods?

People of normal weight typically balance their energy intake throughout the day (or over a few days). If a person eats a large breakfast, he or she will tend to consume fewer calories at lunch, and vice-versa. Meal-to-meal caloric compensation (the ability to regulate energy intake with minimal conscious effort, such as reducing the amount of food consumed on some occasions to compensate for increased consumption at other times) is an important mechanism to avoid excess caloric intake and undesired weight gain. While several studies have shown that fluid calories cause less compensation and therefore may result in the overconsumption of calories, others have yielded opposite or inconclusive results.

At least 62 studies have examined the impact of liquid and solid foods on satiety and energy compensation. The numerous factors that influence satiation must be considered when evaluating this body of literature. They include the amount or volume of food; the food's palatability, consistency, viscosity, and texture; the time the food was administered; the time between the pre-load and the next meal; the subjects' psychological and physiological characteristics; the sample size; and the methods used to measure satiety and consumption. Other critical factors include the subjects' metabolic regulatory systems, such as the blood glucose response to food (Almiron-Roig et al., 2003; Anderson and Woodend, 2003; Mattes and Rothacker, 2001).

Some studies on pre-loading have shown that solids were more satiating than liquids, other studies found the opposite, and yet others found no differential effects at all. A review by Almiron-Roig et al. (2003) summarizes the contradictions in 18 studies. An earlier literature review of 40 pre-load studies by Mattes revealed that dietary compensation for changes in energy intake via fluids is less precise than when solid foods are manipulated (Mattes, 1996). On the other hand, a review by Anderson and Woodend (2003), quantifies the reduction of food intake after pre-loads of various sugars. A study that used a within-subject design in the laboratory showed that, compared with a sugar-containing liquid, a sugar-containing solid had an equal impact on food ingestion if the pre-load periods were the same (Almiron-Roig and Drewnowski, in press 2004).

In recent years, concurrent with the obesity epidemic, satiety studies have examined the effects of increased consumption of energy-containing, nutrient-poor beverages on subsequent intake (Almiron-Roig et al., 2003). Soft drinks are often described as primarily thirst-quenching liquids, but juices and milk are said to be liquid foods with a greater satiating power. One recent within-subjects designed study (n = 32) found that 3 energy-containing beverages (regular cola, 1 percent milk, and orange juice) did not differ in their effects on satiation or the temporal profiles of hunger, fullness, or thirst; they were, however, more satiating than sparkling water (p<0.01) (Almiron-Roig and Drewnowski, 2003).

Some of the confusion results from interactions among physical volume, energy density, and portion size. A controlled study of 36 women found that doubling the volume of a liquid food without changing the energy content significantly decreased the liquid's palatability ratings and increased sensory-specific satiety (p <0.05) (Bell et al., 2003). Another study found that increases in portion size and energy density led to independent and additive increases in energy intake (p<.0001) (Tanja et al., 2004). A further study with 28 lean men found that increasing the volume of a pre-load beverage by incorporating air, independent of energy density, reduced energy intake (p <0.04) (Rolls et al., 2000a).

A few studies have compared calorie compensation in obese and non-obese subjects. Duncan et al. (1983) indicated that obese and nonobese subjects are comparable in their satiety ratio, energy consumption, eating time, and food acceptance. Rolls and Roe (2002) found that energy intake by both lean and obese women were affected by the volume of liquid food infused intragastrically. Further, Rolls et al. (1999a) and Bell and Rolls (2001) found that both lean and obese women were influenced by energy density across all fat contents in food. However, further research is necessary to evaluate eating cues and calorie compensation in obese as well as nonobese subjects.

Studies of children suggest that they respond to dietary energy density and that, although their individual meal intakes are erratic, 24-hour energy intakes are relatively well regulated. These studies also report that children's early learning about food is constrained by their genetic predispositions, including the unlearned preference for sweet and salty tastes, and the rejection of sour and bitter tastes. Evidence of individual differences in the regulation of energy intake has been documented in preschool children. These individual differences in self-regulation are associated with differences in child-feeding practices and with children's adiposity. Initial evidence indicates that imposition of stringent parental controls can potentiate preferences for high-fat, energy-dense foods, limit children's acceptance of a variety of foods, and disrupt children's regulation of energy intake by altering their responsiveness to internal cues of hunger and satiety (Birch and Fisher, 1998).

In summary, the evidence is conflicting that liquid and solid foods differ in their effect on calorie compensation.

SUMMARY

Thirty minutes of at least moderate physical activity on most days provides important short- and long-term health benefits for adults and up to 60 minutes of at least moderate-intensity physical activity might be needed to avoid unhealthy weight gain. Children and adolescents need at least 60 minutes of moderate to vigorous physical activity on most days for maintenance of good health and fitness and for healthy weight during growth. The amount of physical activity that weight-reduced adults need to avoid weight regain is estimated to be from 60 to 90 minutes daily at moderate intensity.

Weight maintenance depends on a balance of energy intake and energy expenditure, regardless of the proportions of carbohydrate, fat, and protein in the diet. To promote recommended nutrient intakes and the adoption of healthy lifestyle changes while losing or maintaining weight, the Committee recommends diets that provide 45 to 65 percent of calories from carbohydrate, 20 to 35 percent of calories from fat, and 10 to 35 percent of calories from protein. Eating foods of low energy density may be a helpful strategy to reduce energy intake when trying to maintain or lose weight. Similarly, limiting the portion size of food eaten or served to others may help control calorie intake.

American Heart Association. Statement on Exercise: benefits and recommendations for physical activity programs for all Americans: a statement for health professionals by the Committee on Exercise and Cardiac Rehabilitation of the Council on Clinical Cardiology, American Heart Association. Circulation 86:340-344, 1992.

Andersen RE, Crespo CJ, Bartlett SJ, Cheskin LJ, Pratt M. Relationship of physical activity and television watching with body weight and level of fatness among children: results from the Third National Health and Nutrition Examination Survey. JAMA 279(12):938-942, 1998.

U.S. Department of Health and Human Services (HHS). Physical Activity and Health: A Report of the Surgeon General. Atlanta, GA: HHS, Centers for Disease Control and Prevention. National Center for Disease Prevention and Health Promotion, 1996.

2 Vigorous physical activity, such as running 5 miles per hour, is any activity that burns more than 7 kcal per min or the equivalent of 6 or more metabolic equivalents (METs) and results in achieving 74 to 88 percent of a person's peak heart rate.

3 The International Association for the Study of Obesity is a professional organization concerned with obesity that works in more than 50 countries around the world. Its membership is drawn from national associations of clinicians, scientists and allied health professionals. As a nongovernmental organization, IASO collaborates with the World Health Organization and other NGOs including the International Diabetes Federation, the World Hypertension League, the World Heart Federation, the International Union of Nutritional Sciences, the International Pediatric Association and the International Federation for the Surgery of Obesity.

4 Moderate physical activity, such as walking 3.5 miles per hour) is any activity that burns 3.5 to 7 kcal per minute or the equivalent of 3 to 6 METs and results in achieving 60 to 73 percent of the person's peak heart rate

5 The International Agency for Research on Cancer (IARC) is part of the World Health Organization. IARC's mission is to coordinate and conduct research on the causes of human cancer, the mechanisms of carcinogenesis, and to develop scientific strategies for cancer control. The Agency is involved in both epidemiological and laboratory research and disseminates scientific information through publications, meetings, courses, and fellowships.

6 The Institute of Medicine is a component of the National Academies. The objective in all their work is to improve decision making by identifying and synthesizing relevant evidence to inform the deliberative process. The Institute provides unbiased, evidence-based, and authoritative information and advice concerning health and science policy to policymakers, professionals, leaders in every sector of society, and the public at large.

7 Position Stands are official statements of the American College of Sports Medicine (ACSM) on topics related to sports medicine and exercise science. Position Stands are based on solid research and scientific data and serve as a valued resource for professional organizations and governmental agencies. Position Stands are first published in the College's scientific journal, Medicine & Science in Sports & Exercise.

8 The negative consequences of high saturated fat and cholesterol intake is discussed in Part D, Section 4 of this report. The negative consequences of low fiber intake is discussed in Part D, Section 5 of this report.

1Moderate physical activities—any activity that burns 3.5 to 7 calories per minute (kcal/min) or the equivalent of 3 to 6 metabolic equivalents (METs) (CDC) and results in achieving 60 to 73 percent of the peak heart rate (ASCM). Other examples include: mowing the lawn or swimming. A person should feel some exertion but should be able to carry on a conversation comfortably during the activity (CDC Web site http://www.cdc.gov/nccdphp/dnpa/physical/terms/index.htm).

1Vigorous physical activities—any activity that burns more than 7 kcal/ min or the equivalent of 6 or more metabolic equivalents (METs) (CDC) and results in achieving 74 to 88 percent of your peak heart rate (ASCM). Other examples include: mowing the lawn with a nonmotorized pushmower and participating in high-impact aerobic dancing. Vigorous-intensity physical activity is intense enough to represent a substantial challenge to an individual and results in a significant increase in heart and breathing rate (CDC Web site http://www.cdc.gov/nccdphp/dnpa/physical/terms/index.htm).